Over the past month or so, a social media campaign has gradually gathered steam that is aimed at convincing advertisers to stop doing business with Breitbart News. It appears that this campaign is now affecting how even large advertising agencies think about the site, which is known for its ties to racist groups.

According to a leaked memo obtained by BuzzFeed, an Australian agency owned by the global advertising giant Omnicom — which handles advertising and marketing campaigns for all of the major Fortune 500 brands including McDonalds and Apple — has advised its staff that many of the firm’s clients are asking that their ads not appear on Breitbart.

The memo mentions the crowdsourced anti-Breitbart campaign known as “Sleeping Giants,” which encourages Twitter users and those on other social networks to shame large companies into not doing business with Breitbart. The latest update says that over 1,000 advertisers have blocked the site from their media-buying programs.

A recent check of the Breitbart site showed there are few if any ads for large consumer brands. Most of the advertising appears to consist of ads for diet supplements and conservative newsletters, among other things.

THE SG UPDATED CONFIRMED LIST: https://t.co/4TYRUOj9sv
THE SG FAQ: https://t.co/wfSUQBjOgj
REMOVE ADS YOURSELF: https://t.co/V4V8qWyyv3 pic.twitter.com/JszlNFryKY

— Sleeping Giants (@slpng_giants) December 16, 2016

The Omnicom memo said that “brands who advertise on Breitbart [are] being called out by a Twitter following called Sleeping Giants,” according to BuzzFeed’s report. “Many of our global clients have already requested that we block this site.”

The memo goes on to point out that while the content on the Breitbart site “is pretty unpalatable for most, it isn’t illegal or prohibited by most exchanges” such as Google’s display ad network. But the Omnicom letter advises staff to make sure that any ad buys they do through ad networks don’t include the Breitbart site.

Breitbart is known for supporting what some refer to as the “alt right” movement in the United States, a loose affiliation of politically conservative groups that include overtly racist organizations such as Richard Spencer’s National Policy Institute, which advocates for something it calls “white nationalism.”

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One of Breitbart’s most notorious writers is Milo Yiannopolous, who has been banned from Twitter for encouraging his followers to abuse and harass other users including the comedian and actress Leslie Jones.

The Breitbart writer, who helped popularize the term “alt right,” also recently had a major book contract with Simon & Schuster pulled because of comments he made during an interview, in which he appeared to condone sex between adult men and 13-year-old boys.

There were reports that Breitbart News might be considering severing its ties to Yiannopolous because of his comments, after a number of the site’s employees complained and threatened to quit. On Tuesday, the Breitbart writer announced that he would no longer be affiliated with the site.

What remains to be seen is whether the response from advertisers and agencies like Omnicom will increase the pressure on the site to moderate its coverage.

In November, the digital ad platform AppNexus said that it was banning Breitbart from using its ad tools because the site promoted racism and content that might incite violence, which was against the firm’s rules.

Food maker Kellogg Co. announced several months ago that it would no longer place advertising on Breitbart’s site, at which point the publisher launched a hashtag campaign called #DumpKelloggs. The site’s editor-in-chief said at the time that boycotting Breitbart News “for presenting mainstream American ideas is an act of discrimination and intense prejudice.”